An NIPT test is a blood test that screens a fetus for the most common chromosomal defects, including Down Syndrome, trisomy 13, and trisomy 18, as well as other sex chromosome abnormalities. (MaterniT21, for example, is a common NIPT screen.) Also referred to as cell-free DNA screening, the blood test can be performed in women who are at least nine weeks into their pregnancy. While it is routinely offered to women with high-risk pregnancies, meaning the women are at least 35 years old, have had a child with a chromosomal disorder, or have had an ultrasound that showed a potential problem with their fetus, many obstetricians are now offering the test to all women, regardless of age or pregnancy risk.

How NIPT Works

A pregnant woman's blood contains both maternal DNA and cell-free fetal DNA, which is DNA from the placenta that has crossed into the mother's bloodstream. NIPT measures the fragments of this circulating fetal DNA. If the test detects abnormal amounts of certain DNA, then the screening would come back positive. For example, high amounts of chromosome 21, 13, or 18 suggest that the fetus may have an extra copy of one of those chromosomes in each cell in the body, rather than the usual two copies. Having an extra 21 chromosome, for instance, can lead to Down Syndrome, a disorder characterized by intellectual and physical disabilities, including cognitive delays, heart defects, and an increased risk of developing Alzheimer's disease.

What has doctors, researchers, and women with high-risk pregnancies excited about NIPT is its accuracy compared to the maternal serum tests previously offered, which, while also noninvasive, are not nearly as accurate an NIPT test.

"One out of 20 women who had a maternal serum screen for Down Syndrome would get a false positive, whereas 1 out of 1,000 women getting NIPT would get a false positive," says Blair Stevens, M.S., CGC, a genetic counselor at the University of Texas Health Science Center in Houston.

Knowing your baby's risk of having a chromosomal defect with a blood test can help you avoid more invasive tests. With that said, keep in mind that NIPT is a screening tool; it is not diagnostic. A positive test result signals that you will have to undergo a more invasive diagnostic test, such as an amniocentesis or chorionic villus sampling, which carry a small risk of miscarriage. Only those tests can definitively confirm or rule out a chromosomal disorder.

NIPT Pros and Cons

While an NIPT test has benefits, it does have limitations. Women with low-risk pregnancies have far more frequent false positives than women who are considered high risk. This can lead to unnecessary diagnostic procedures. In addition, a negative result does not guarantee that a fetus harbors no chromosomal abnormality, and NIPT does not test for all chromosomal disorders.

Aside from its ability to detect disorders, it can also predict fetal sex some 10 weeks earlier than the standard ultrasound and with a 99 percent accuracy. This has many soon-to-be parents excited, but Stevens offers words of caution for parents seeking NIPT solely to discover the sex of the baby.

"We have had unfortunately several patients come to our office with abnormal results who were very confused and very regretful for having the test," says Stevens. They wanted a straightforward gender blood test but ended up with devastating news they had not been prepared to receive. Stevens recommends that parents seek genetic counseling both before and after testing to make sure they understand what they are signing up for when they undergo prenatal testing.